I wouldn't make plans to ride solo with any guy if I had a serious boyfriend (in my case, I have a serious husband!). So regardless of whether he's hitting on you or not, I'd tell him you'd love to do a group ride with him, but not alone. If he laughs and says it's "just a ride" and he's not interested beyond that, I'd still stick to my guns and say I don't ride alone with guys unless they're "my guy". Same reason I wouldn't do just about anything alone with another guy ... go to lunch, etc.

I had a guy team up with me once when I was riding by myself. He came up behind me, started chatting with me and rode with me for several miles. Said it was safer to ride together. He was a cop, so he seemed extra protective even, making sure I was aware of cars turning right ahead of us, etc. It was a little awkward, but it wasn't a planned thing and he never hit on me. If he had asked to plan another ride together, I would have turned him down, but since it just sort of happened while we were riding, I let it go.

Anyways, those are my two cents.